1. Field of the Invention
The invention relates to a method of producing a battery including a nonaqueous electrolyte (a nonaqueous secondary battery).
2. Description of Related Art
A nonaqueous secondary battery such as a lithium-ion battery or the like is lighter in weight and higher in energy density than a conventional battery. Thus, in recent years, nonaqueous secondary batteries are preferably used as vehicle-mounted high-output power supplies or the like. In the production of batteries of this kind, it is typical that an electrode body is first made of a positive electrode and a negative electrode and then a battery assembly is constructed with the electrode body and a nonaqueous electrolyte. Then, a specified initial charge (conditioning) and aging under high temperatures are performed to the battery assembly thus constructed. Thereafter, the performance is tested (e.g., a self-discharge test).
In the self-discharge test, the generation of a minute internal short-circuit in the battery assembly is determined by leaving the battery assembly having an adjusted state of charge (SOC) alone for a specified period of time and measuring a voltage drop amount during this period (self-discharge). However, it is sometimes the case that, even after adjustment of the SOC, a voltage of the aged battery assembly is not stabilized for a while and continues to rise or fall (hereinafter, this period will be simply referred to as a “voltage instability period”). For that reason, in order to accurately conduct a test (determination), there is a need to wait until the voltage of the battery assembly is sufficiently stabilized. In the production method of the related art, the time required for production (typically, the initial period of the self-discharge test, i.e., the voltage instability period) tends to become longer. Examples of a technique for addressing this problem include Japanese Patent Application Publication No. 2012-084346 (JP 2012-084346 A) which recites that a self-discharge test period (particularly, a voltage instability period) can be shortened by forcedly discharging an aged battery after the battery is kept in a high SOC state for 1 to 7 days and by setting a difference in voltages available before and after the forced discharging to fall within a predetermined range.
However, the research conducted by the inventors revealed that, in the method disclosed in JP 2012-084346 A, the aforementioned battery voltage fluctuation often lasts for a long period of time (e.g., 5 days or longer) depending on, e.g., the condition of the electrode lot or an aging condition. This tendency was particularly conspicuous in a battery which is required to generate high output power in a low SOC region (e.g., a region where an SOC is 30% or less), e.g., a battery mounted to a plug-in hybrid vehicle (PHV). Accordingly, from the viewpoints of productivity, work efficiency and costs, there is a demand to further shorten a self-discharge test period (e.g., a voltage instability period) without reducing test accuracy.